The present invention relates to a coating device and more especially to a coating device having an applicator mechanism for applying a liquid layer to a sheet-type support material which is conducted over an applicator roll.
The development in coating technology leads to assemblies of ever higher performance, i.e., to higher processing speeds and to thinner coating films or more highly concentrated solutions of the coating agents, which is desirable above all for ecological and economic reasons.
These demands can be met only by a substantial increase in the precision of the interacting components, above all by maintaining a uniform coating gap between the applicator roll and applicator mechanism. For high coating speeds, smaller gap distances are required, and it is indispensable in this case to maintain precise distances. Radial deviations of the applicator roll cause periodic differences in layer thickness, which are undesirable in every case.
The mounting of the applicator roll in this context requires particular attention, since high accuracy of running in the axial and radial directions is obtained by well-designed mounting. The running accuracy is here defined as the deviation from the ideal axis of rotation, the deviation arising as the total error from all the interacting components on the applicator roll. The deviation of rolling bearings having a particularly high accuracy cannot be less than 3 .mu.m from the ideal axis of rotation. This minimum theoretical radial deviation of such rolling bearings is increased by further individual errors of the applicator roll. During the rotation of the applicator roll, the errors of the left-hand side thereof can lead to a diminution of the distance between the applicator roll and the coating device, while at the same time an increase in the distance between the applicator roll and the coating device results on the right-hand side.
Furthermore, due to the traction forces of the sheet-type support material being coated, the inevitably-present bearing play leads to a misalignment of the applicator roll, whereby an enlargement of the gap results. Likewise, different traction forces on the right-hand and left-hand sides of the sheet-type support material can lead to tumbling of the applicator roll during operation of the coating device, whereby achievement of a constant distance between the applicator roll and the coating device is limited.
Further deteriorations are to be expected, for example, during prolonged operation of the coating device, due to temperature variations in the region of the applicator mechanism, or due to wear in the mounting of the applicator roll.
Known applicator systems for applying a coating to printing plates, manufactured by Hoechst AG, are mounted to be displaceable in parallel guides relative to the applicator roll. A change in the distance between the applicator roll and the application lip can be effected by means of stops, in order to apply different coating thicknesses to the support material. Great efforts are made here in the design, in order to achieve precise dimensions of the gap and to maintain the layer thickness across the entire width of the support material within very narrow tolerances in the .mu.m range. The radial deviation of the applicator roll and the play of the mounting on both sides and the radial deviation thereof are decisive for the accuracy of the layer thickness. Even with a most careful selection of the bearings and precise machining of the applicator roll, in order to achieve a high truth of running, the set layer thickness cannot be kept constant due to varied material stresses, increases in the play of the bearings and temperature influences. This leads to products of lower quality, higher reject rates, increased testing costs or, due to exchange and reworking of the components concerned, to high down-time costs and fabrication costs.
The device, described in DE-A 3,309,343, for applying at least one coating layer to a moving, sheet-type substrate has a rotating applicator roll which guides the substrate. A curtain coater is arranged close to the applicator roll, separated only by a gap. The coating gap can be changed by adjusting the curtain coater with a radial component. The coater can also be adjusted in any other directions desired, in order to take account of the numerous factors which must be watched in operation, such as the material properties of the emulsion and overcoating solutions, the layer thickness on the sheet-type support material, the range of the coating speed, and the like. However, no precautions are taken to allow for non-uniformities of the layer thickness due to radial deviation of the applicator roll nor due to play in the mounting of the coater.
In European Patent EP-B 0,168,986, a sliding film coating at reduced pressure with tangential application of the coating solution is described. The sliding film flows down on a vertical wall and passes through a gap onto the sheet to be coated. At this point, the sheet is deflected over a roll in such a way that the coating film comes tangentially into contact with the sheet. The coating device is mounted to be rotatably adjustable about the axis of the coating roller, and the angle of the sliding track is adjusted within a range from 20.degree. to 120.degree. relative to the horizontal. For this purpose, the housing of the coating device is supported on each side by arms which are rotatable about the axis of the coating roller. The support arms allow the coating device to be arranged in any desired position along the periphery of the coating roller. Even in this known coating device, no precautions are taken to compensate radial deviations or mounting errors, which impair the constancy of the layer thickness on the support material which is to be coated.